Another electrical question...


bufordtpisser

New Member
Re: Another electrical question...

hertig - 3/23/2007 11:30 PM

Really? I would think that if you change the resistance between the meter leads, the voltage (due to current) across those leads would change to compensate (we will assume the current is essentially constant). And so if the reading is driven by voltage,as would be implied by the need for a shunt, the reading will be changed even if the current is the same. Last I heard, it was a law that Voltage = Current x Resistance. Or has the ACLU overturned that one too?

If there is an internal shunt, it will have a resistance calibrated to provide a 'current' reading which is accurate. If you put another resistance in parallel with that resistance, the value will change. For instance if you have 2 ohms there already, and put another 2 ohms in parallel, the effective resistance will be 1 ohm, or half what it 'should' be. Thus the voltage will be half what it was before for any particular current, and so the current reading will be 1/2 of what it should be.

That was my mistake. What I should have said was that if you do not know if there is an internal shunt, wire one in and then test the guage with a meter to see if the reading is correct. The use of the dropping resister is after the guage, and therefore will not have an effect on the reading. As far as 60 amps not being an issue in a DC circuit, I have had readings many time that amount on DC circuits. And even though the wire were very hot at the time, other components failed long before the guage crapped out. I have also seen severe damage in circuits where the current drawing device was shutdown suddenly and backfeed current literally melted components. Did you ever wonder why most circuits on a vehicle had 20 to 30 amp fuses on wires as small as 14 guage?? The resister that I was suggesting was merely to stop back feed. The proper way to do amperage measurement is to use a meter that is not a part of the circuit. Either using a clamp on meter, or using a pickup coil that the wire passes through that will allow an amperage reading without the meter being an active part of the circuit. I was not trying to rain on anyones parade, I merely did not accurately explain my answer. I have 10 years as a senior service tech for an electric motor and generator shop under my belt and sometimes I guess that I just assume that others that are trying to do these types of projects know more than they actually do. I will try to remember that in the future.
 

TexasClodhopper

Senior Member
Re: Another electrical question...

No harm done or meant, Marvin. We're all amateurs here about something, so we have varying levels of understanding and many levels of expression. It's hard to know how much detail to put into an answer.

I'm sure woodworker1 would have trouble explaining to us how to make a double-jointed backbone miter dovetail joint in particle board, now wouldn't he?
 

woodworker1

New Member
Re: Another electrical question...

Uh, when it comes to wiring something, trust me...you can't take any knowledge for granted with me.
Thanks for all the replies.
 

hertig

Senior Member
Re: Another electrical question...

Why would you want to do anything with particle board? That stuff is inferior for most applications. And can give off noxious fumes to boot.
 

woodworker1

New Member
Re: Another electrical question...

Yeah, I agree. I don't even use it for counter top underlayment anymore. Too heavy and makes too much of a mess.
 
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